The present invention generally pertains to the fields of reticle technology, photolithography, and metrology. More specifically, the present invention relates to an efficient means of measuring a standard critical dimension feature and insuring that this feature is representative of cross-chip average critical dimension size.
There is a need to make in-line measurements of feature sizes which are both efficient in measurement execution and simultaneously representative of feature sizes across an entire chip. If the in-line measurement is not representative of the average feature size, then an in-line monitor may indicate that a wafer is right on target, but when the wafer reaches end-of-line testing, the electrical parameters, incorporating all features within the chip, may be found to be shifted away from target. There are a number of existing approaches which attempt to solve this problem, but each of these approaches are lacking in one or more respects.
One existing approach to solve this problem is to measure a standard critical dimension (“CD”) feature at a small number of points across a wafer. This approach or method is the industry standard. It works very well if the standard CD feature is the same size as the cross-chip average CD size. However, if the standard CD feature does not match the cross-chip average, then bringing the standard CD feature onto target will result in the cross-chip average CD being off of target.
A second existing approach to solve this problem is to measure a standard CD feature at a small number of points across a wafer, but target this feature such that the cross-chip average feature size is on target. For example, if the standard CD feature is ten (10) nanometers larger than the cross-chip average feature size, then adjust the dose such that the standard CD feature ends up ten (10) nanometers above target, so that the cross-chip average feature size ends up right on target. This approach or method results in efficient measurements and also results in cross-chip average CD values on target. However, it is typically impractical to implement because each reticle has the potential to require a different CD target. This makes it very cumbersome to determine if a given measurement is within specified limits or not, since the specified limits may be unique for that reticle rather than a global for a given technology. It also hinders statistical process control, since different CD targets may result in a control chart with data points jumping up and down, appearing out of control but in reality simply reflecting a wide range of CD targets.
A third existing approach or method to solve this problem is to search within the chip for a feature which happens to be at the cross-chip average feature size. This feature is then measured at a small number of points across the wafer. This technique results in cross-chip CD values being on target. However, it means that each reticle will need to have a different feature measured at the metrology step. This will considerably slow the metrology set-up as the suitable feature is located. The non-standard CD feature will also make it very difficult to determine if a measurement is incorrect due to the metrology tool measuring the wrong feature, since it is much harder to distinguish correct features from incorrect features when even the correct features are non-standard.
A fourth existing approach or method to solve this problem is to measure a large number of features within a chip and then repeat this for a small number of chips across a wafer. This method yields cross-chip average CD values on target. However, it is highly inefficient for the metrology process, and severely restricts the metrology throughput.
The learning curves for any of the four aforementioned approaches or methods may be shortened by first measuring the specified features for that approach on the photomask rather than on the wafer. The results can then be used to predict the correct exposure does through a calibration curve comparing photomask feature size to applied dose and wafer feature size for previous photomasks or for a reference photomask with multiple feature sizes. This option thus pre-compensates the dose for the photomask-to-wafer size bias, improving the chances that the first wafer printed will be on target and will not need to be reworked. While this method does reduce the likelihood that the first wafer printed will be off-target, it does not change any of the negative aspects of the four above-identified approaches as they relate to in-line monitoring of production wafers. The following issues still impact these approaches: having a standard CD feature which is sized differently than the cross-chip average feature size; impractical targeting requirements; metrology recipes which are slow to set up and unreliable in execution; and an excessive number of measurements on production wafers.
Therefore, an improved method of making in-line measurements of feature sizes, which is both efficient in measurement execution and simultaneously representative of feature sizes across an entire chip is needed. The present invention provides such a method. Features and advantages of the present invention will become apparent upon a reading of the attached specification, in combination with a study of the drawings.